Animals with the biggest brains and broadest diets display the most self-control, according to the results stemming from the largest study of animal intelligence to date.

Self-control in animals is not quite the same as it is for humans. Animals don't count calories or worry about drinking too much. But self-control is important for survival throughout the animal kingdom, be it exhibited in a predator that waits to pounce on its prey at the right moment, or a hungry mother bird that restrains herself from eating all of a catch in order to feed her young.

The research involved two experiments, both of which were designed to assess an animal's inhibitory control, which is a measure of brain function associated with the ability to control impulses and delay gratification.

One test involved a piece of fruit hidden in an opaque plastic cylinder with openings at both ends. The animals placed facing the fruit were trained to access the food by moving to one end of the cylinder and gaining access via the openings. The animals exhibit self-control by resisting the impulse to reach out for unattainable food directly in front of them, the researchers said.

Another test involved placing a treat underneath one of three cups and letting the animal watch on as the position of the cups was rearranged. To access the food, the animals had to show self-control by not searching under the first cup and looking for food beneath the other cups instead.

On both tests, large primates such as gorillas, bonobos and orangutans performed well, gaining access to the food about 90 percent of the time, much better than smaller primates such as squirrel monkeys or Coquerel's sifakas, which succeeded less than half of the time.

The researchers report that the species with the highest scores had the biggest brains. However, while absolute brain size mattered, relative brain size did not, which suggests that species' differences in self-control may have more to do with how the brain is wired than with how big a brain is compared to overall body size.

Additionally, the researchers found that primates with superior self-control also had the most diverse diets, requiring them to forage in different places during different seasons. An animal that relies on a variety of seasonal foods cannot always rely on the same spot to forage, which is an exhibition of resisting the tendency to return to the same place for food, the researchers said.

The research was led by Evan MacLean, Brian Hare and Charlie Nunn of Duke University, with the help of zookeepers around the world who conducted feeding experiments at the study authors' request.